1. Field of Invention
This invention relates to position transducers. More specifically, this invention is directed to a position transducer system that can be calibrated for short range errors, without using an external position reference.
2. Description of Related Art
Various movement or position transducers for sensing linear, rotary or angular movement are currently available. These transducers are generally based on either optical systems, magnetic scales, inductive transducers or capacitive transducers.
FIG. 1 shows a generic transducer 100 having a scale 120, scale elements 122, a read head 110 and read head elements 112. The transducer may be optical, capacitive, magnetic or inductive.
The transducer 100 outputs two signals S.sub.s and S.sub.c that vary sinusoidally as a function of the position of the read head 110 relative to the scale 120 along a measuring axis 130. The signals S.sub.s and S.sub.c are basically identical except for a quarter-wavelength phase difference between them, as illustrated in FIG. 2. The transducer electronics use these two signals to derive the instantaneous position of the read head 110 relative to the scale 120 along the measuring axis 130.
Ideally, the signals S.sub.s and S.sub.c are perfect sinusoids with no DC offsets, have equal amplitudes, and are in exact quadrature (i.e., a quarter-wavelength out of phase relative to each other, also referred to as "orthogonal" herein). In practice, the signals S.sub.s and S.sub.c have small DC offsets, their amplitudes are not equal, and they have some orthogonality error. In addition S.sub.s and S.sub.c may have distorting harmonic components. In addition, the transducer electronics can introduce offset, gain, and non-linearity errors.
Calibrating the transducer 100 and compensating for these errors requires determining the DC signal offsets, the amplitudes of the fundamental signals S.sub.s and S.sub.c, the non-orthogonality (i.e., the phase error) between the fundamental signals, and the amplitudes of the harmonic components. There are two commonly used prior transducer calibration methods.
The most common method is the "lissajous" method. The lissajous method typically comprises inputting the two nominally orthogonal readhead signals to an oscilloscope, to drive the vertical and horizontal axes of the oscilloscope. The read head is continuously scanned relative to the scale to generate changing signals. The oscilloscope display is observed, and the readhead is physically and electronically adjusted until the display indicates a "perfect" circle, centered at zero on both axes. Under this condition, the amplitude, orthogonality and offset of the two signals are properly adjusted.
The lissajous method assumes that the two signals are both perfect sinusoids. Typically, there is no adjustment for harmonic errors which can distort the circle, as it is hoped that these are made insignificant by fixed features of the transducer design and assembly. The lissajous method is well-known to those skilled in the art, and has been performed by sampling the two signals with computer-based data acquisition equipment.
Alternatively, it has also been common to accept any transducer errors due to imperfect amplitudes, orthogonality, harmonics and offsets, and to use an external reference, such as a laser interferometer, to accurately correct position errors from the read head 110 at the system level, at predetermined calibration positions relative to the scale 120.
Position transducers typically require initial factory calibration, and periodic calibration or certification thereafter. In both cases there is a cost for the associated equipment and labor. When the transducer 100 is located in a remote location, it is difficult to set up the external data acquisition equipment and/or accurate external reference required for calibration. As a result, the transducer 100 often has to be transported to another site or shipped back to the factory for calibration. This results in long down time and increased costs.
Even in cases where the transducer 100 does not have to be transported for calibration, the special tools and increased time required to set up the external display and/or reference result in increased costs and down time. Thus, calibration and recalibration is often minimized or avoided, in practice. Since most practical position transducers are sensitive to variations during production, installation, and use, measurement errors normally increase in the absence of calibration, and in the periods between recalibration.